I did notice few months ago that it's not possible to open iTunes tracks by dragging the proxy to any dock icons of audio apps. (Not only to AP.)
I don't know what it caused, because it started somewhat suddenly while I was still using iTunes 9 on AP 1.5.5 on Leopard, i.e. there was actually no change on my side of things.
Eventually I've updated to the iTunes 10.5 bloatware few weeks ago (an iPad 2 made it necessary), and that didn't fix it.

The workaround is to hit cmd-shift-R to reveal the iTunes track in Finder and than you can drag the file to AP (or any other app).

But why did it stop working while actually nothing has changed on my working system? (Leopard 10.5.8, iTunes 9, AP 1.5.5 and several other similar apps like MediaRage, Fission, WaveEditor, etc.) All that I've ever installed on Leopard since quite some time was a bunch of security updates.

But why did it stop working while actually nothing has changed on my working system? (Leopard 10.5.8, iTunes 9, AP 1.5.5 and several other similar apps like MediaRage, Fission, WaveEditor, etc.) All that I've ever installed on Leopard since quite some time was a bunch of security updates.

I'm pretty sure it was either from an update to iTunes, or to AP itself when I lost the drag functionality. Could have been different for you I suppose, but it seems likely that one problem update happened without you being fully conscious of it.

It is very easy to import a song from iTunes: simply open iTunes and drag the song from iTunes onto the AmadeusPro icon in the dock. This will open the corresponding file and allow you to edit it! Simple.